1. Field of the Invention
This invention relates to position loop control systems, and, more particularly, to numerical control systems for moving a large machine along a single axis.
2. Description of the Prior Art
Position loop numerical control systems, more particularly phase analog position loop control systems, have been used to control the movement of large machines along a single axis. Each side of the large machine would be attached to a rack wherein each rack is driven by a pinion that is mechanically coupled to a motor. Each motor is driven by a separate phase analog position loop control system, wherein the same commanded position signal is used to command movement of each side of the machine. Transducers, used to detect the actual position of the motor and/or machine, are provided to feed back a signal that is indicative of the actual position of each side of the machine. The commanded position signal is then compared with each signal indicative of the actual position of each respective side of the machine. The comparison results in the generation of position error signals which are applied to respective servo amplifiers to cause each side of the machine to be driven toward the commanded position. In view of the different dynamic characteristics of each phase analog loop, the motors being driven, the mechanical parts being moved, etc., each side of the machine does not ordinarily move at the same pace toward the commanded position. It would be undesirable to allow one side to move much faster than the other side in view of the great stress it could cause to the control system, to the machine being moved, and to the mountings for the machine. It would thus be desirable to monitor the actual movement of each side of the machine. The system can then be shut down if the difference in position between each side becomes greater than an allowable amount. Additionally, the drive to the slower moving side can be increased, and/or the drive to the faster moving side can be decreased. In the past, the only way in which this object could be carried out would be to use separate position transducers for each side of the machine, and provide a phase discriminator, which would include a digital to analog converter, for generating an output signal having a magnitude indicative of the difference in actual position between each side of the machine and a polarity indicative of which side of the machine has moved ahead of the other side. The cost of this circuitry is almost as great as that required to provide a completely separate phase analog position loop system.